This is a rather extensive list of nature/science museums in Colorado. I include the location for each as they can serve not only as a resource for you as a teacher, but as a way to bring realia into science classrooms, or rather, bring students to the realia. This gives you the opportunity to explore different types and prices of museums to find what may work best for your class.
http://www.walkingmountains.org/
Alamosa)
of Denver)
Florissant)